Operational aerosol retrieval at subkilometer resolution using
OceanSat-2 OCM over land: SAR algorithm, uncertainties, validation &
inter-sensor comparison.
Abstract
The OCM sensor onboard OceanSat-2 is providing data in visible and NIR
bands. Due to the limited spectral coverage of OCM, the widely used
dark-target and deep-blue aerosol algorithms cannot be adapted. Here, a
new aerosol optical depth (AOD) retrieval algorithm for OCM (or similar
sensors) over land, termed SAC Aerosol Retrieval (SAR) is described. It
utilizes two blue bands for the AOD inversion and NIR band to
characterize the surface in visible bands without assuming red and NIR
bands transparent to aerosols. Unlike the dark-target algorithm, the SAR
algorithm can retrieve AOD over bright arid and urban areas too. The
uncertainty analysis of SAR suggests a theoretically expected error (EE)
envelope of ±(0.06+0.26×AOD) for typical retrieval conditions. OCM AOD
over land is retrieved operationally for the first time over Indian and
neighboring countries’ landmass at the finest spatial resolution of
0.0070. The SAR algorithm is validated against in-situ
AOD measurements during the years 2016-2018 at 21 AERONET stations
located in south-Asia. Overall validation using 1900 match-up points
shows correlations exceeding 0.8 with 74% of retrievals within EE. The
retrievals over cropland, grassland, and mixed land cover types show
high (low) correlation (bias), while over bright urban areas somewhat
low (high) correlation (bias) is observed. Excluding monsoon season, OCM
AOD retrievals show good performance over the year. The performance of
MODIS dark-target and OCM AOD, against common in-situ, is close to each
other. The study shows that OCM AOD can be used for air quality
monitoring/modeling at high spatial resolution.